Giovanni Paternoster

State of the art timing in TOF-PET detectors with LuAG, GAGG and L(Y)SO scintillators of various sizes coupled to FBK-SiPMs

Time of flight (TOF) in positron emission tomography (PET) has experienced a revival of interest after its first introduction in the eighties. This is due to a significant progress in solid state photodetectors (SiPMs) and newly developed scintillators (LSO and its derivatives). Latest developments at Fondazione Bruno Kessler (FBK) lead to the NUV-HD SiPM with a very high photon detection efficiency of around 55%. Despite the large area of 4×4 mm2 it achieves a good single photon time resolution (SPTR) of 180±5ps FWHM. Coincidence time resolution (CTR) measurements using LSO:Ce codoped with Ca scintillators yield best values of 73±2ps FWHM for 2×2×3 mm3 and 117±3ps for 2×2×20 mm3 crystal sizes. Increasing the crystal cross-section from 2×2 mm2 to 3×3 mm2 a non negligible CTR deterioration of approximately 7ps FWHM is observed. Measurements with LSO:Ce codoped Ca and LYSO:Ce scintillators with various cross-sections (1×1 mm2 - 4×4 mm2) and lengths (3mm - 30mm) will be a basis for discussing on how the crystal geometry affects timing in TOF-PET. Special attention is given to SiPM parameters, e.g. SPTR and optical crosstalk, and their measured dependency on the crystal cross-section. Additionally, CTR measurements with LuAG:Ce, LuAG:Pr and GGAG:Ce samples are presented and the results are interpreted in terms of their scintillation properties, e.g. rise time, decay time, light yield and emission spectra.

NUV Silicon Photomultipliers With High Detection Efficiency and Reduced Delayed Correlated-Noise

In this paper, we present the characteristics and performances of new silicon photomultipliers (SiPMs), produced at FBK, for the near-ultraviolet (NUV) light detection, with reduced afterpulsing and delayed optical crosstalk. To study these components of the correlated noise, we manufactured SiPMs on silicon wafers featuring different substrate minority-carrier lifetime. This parameter proved to be crucial in determining the amount of delayed optical crosstalk and afterpulsing caused by photo-generated carriers diffusing from the substrate to the cell active region. With a very low substrate lifetime, we were able to minimize this correlated noise component to few percent at room temperature. Besides reducing the excess noise factor, the lower delayed correlated noise allows biasing the SiPM at higher voltages, reaching higher values of photon detection efficiency.

Performance of NUV-HD Silicon Photomultiplier Technology

In this paper, we present the full characterization of a new high-density (HD) cell silicon photomultiplier (SiPM) technology for ultraviolet (UV) and blue light detection, named near UV HD SiPM. Thanks to an optimized border region around each cell, we were able to develop devices having a very high detection efficiency and, at the same time, a high dynamic range. We produced SiPMs with a square cell pitch of 15, 20, 25, and 30 μm featuring a peak efficiency in the violet region ranging from 40% to 55%, according to the cell size. We tested this technology for time-of-flight positron emission tomography. Using two 4 × 4 mm2 SiPMs with a 25 × 25 μm2 cell pitch coupled to 3 × 3 × 5 mm3 LYSO scintillators, we reached for the first time 100-ps full-width at half-maximum coincidence time resolution. This result was independent of the temperature in a range from 20 °C to -20 °C. At the same time, thanks to the high dynamic range and low correlated noise, we obtained an energy resolution lower than 9% for 511-keV γ-rays.

Modeling and Characterization of Antireflection Coatings with Embedded Silver Nanoparticles for Silicon Solar Cells

Plasmonics applied to solar cells is a widely investigated research field. Its main purpose is to include plasmonic structures in the cell design, in order to increase light trapping in the cell and, consequently, its energy conversion efficiency. Light scattering by plasmonic structures has been extensively studied by depositing metal nanoparticles on both sides of the cell, in order to enhance the transmission into the cell and/or the path length of the transmitted radiation. The effects due to the nanoparticles were studied also in the presence of dielectric layers covering the cell and working as anti-reflective coatings (ARC), although a complete discussion on the possible optimization of this setup is lacking. In this work, we provide a joint computational and experimental investigation of the optical properties of silver nanoparticles embedded in a SiO 2 ARC located on top of a crystalline silicon wafer. The effect of the particle size, particle position within the ARC layer, and surface coverage on the light transmitted to the silicon crystal are simulated by a finite-difference time-domain (FDTD) in-house software. On the experimental side, a composite anti-reflective structure, made of a silica layer with embedded silver nanoparticles, is deposited on top of silicon wafers. Samples differing in the size and position of the embedded metal particles are produced. For each configuration, the total reflectance is optically measured by means of a photo spectrometer coupled to an integrating sphere. We provide direct comparison of experimental and simulation results, along with an exhaustive discussion about the transmission efficiency of the investigated systems. We also discuss how our analysis might be extended to different configurations and cell design.

Performance of FBK low-afterpulse NUV silicon photomultipliers for PET application

The improvement of the coincidence resolving time (CRT) is one of the key factors for the next generation of positron emission tomography (PET) scanners. Silicon photomultipliers (SiPMs) are strong candidates to substitute photo multipliers tubes because of their compactness, ruggedness and insensitivity to magnetic fields. In order to achieve the best CRT, the SiPM should have high PDE which can be obtained increasing the bias voltage. We recently improved the NUV SiPM technology, with the addition of a new substrate type that provides significantly lower afterpulsing probability (Low-AP). This enables to extend the maximum bias voltage and thus obtain higher PDE. Additionally, we implemented a lower electric field version (Low-F) to reduce the field-enhanced thermal generation components of the dark count rate. In this work we present results of energy and timing resolution for PET application, using LYSO scintillator crystals, and coupled with 3 × 3 mm2 NUV SiPMs of three types: non-Low-AP, Low-AP and Low-AP + Low-F. All the devices reach very similar energy resolutions, around 9.5 %, and close to the intrinsic limit of the LYSO. Concerning the timing resolution, we found that the Low-AP substrate achieves an improvement of the CRT of ≈ 30 ps, confirmed with the Low-F. Using 4 × 4 mm2 Low-AP SiPMs coupled to 3 × 3 × 5 mm3 and 3.8 × 3.8 × 22 mm3 LYSO crystals we obtained CRTs of 130 and 200 ps FWHM, respectively.

Set-up and methods for SiPM Photo-Detection Efficiency measurements

In this work, a compact set-up and three different methods to measure the Photo-Detection Efficiency (PDE) of Silicon Photomultipliers (SiPMs) and Single-Photon Avalanche Diodes (SPADs) are presented. The methods, based on either continuous or pulsed light illumination, are discussed in detail and compared in terms of measurement precision and time. For the SiPM, these methods have the feature of minimizing the effect of both the primary and correlated noise on the PDE estimation. The PDE of SiPMs (produced at FBK, Trento, Italy) was measured in a range from UV to NIR, obtaining similar results with all the methods. Furthermore, the advantages of measuring, when possible, the PDE of SPADs (of the same technology and with the same layout of a single SiPM cell) instead of larger devices are also discussed and a direct comparison between measurement results is shown. Using a SPAD, it is possible to reduce the measurement complexity and uncertainty since the correlated noise sources are reduced with respect to the SiPM case.

Study of the photo-detection efficiency of FBK High-Density silicon photomultipliers

This work presents a study of the factors contributing to the Photo-Detection Efficiency of Silicon Photomultipliers (SiPMs): Quantum Efficiency, Triggering Probability and Fill Factor. Two different SiPM High-Density technologies are tested, NUV-HD, based on n-on-p junction, and RGB-HD, based on p-on-n junction, developed at FBK, Trento. The quantum efficiency was measured on photodiodes produced along with the SiPMs. The triggering probability, as a function of wavelength and bias voltage, was measured on circular Single Photon Avalanche Diodes (SPADs) with 100% fill factor. Square SPADs, having the same layout of single SiPM cells, were studied to measure the effective fill factor and compare it to the nominal value. The comparison of the circular and square SPADs allows to get the transition region size between the effective active area of the cell and the one defined by the layout.

Shashlik Calorimeters With Embedded SiPMs for Longitudinal Segmentation

Effective longitudinal segmentation of shashlik calorimeters can be achieved taking advantage of the compactness and reliability of silicon photomultipliers. These photosensors can be embedded in the bulk of the calorimeter and are employed to design very compact shashlik modules that sample electromagnetic and hadronic showers every few radiation lengths. In this paper, we discuss the performance of a calorimeter made up of 12 such modules and able to sample showers every

Characterization of Linearly Graded Position-Sensitive Silicon Photomultipliers

\sim 4X_{0}

Characterization of high density SiPM non-linearity and energy resolution for prompt gamma imaging applications

. In summer 2016, this prototype has been exposed to electrons, muons, and hadrons at CERN PS (East Area T9 beamline). The performances in terms of energy resolution, linearity, response to minimum ionizing particles, and reconstruction of the shower profile are discussed.